Idling issues with '97 Sentra

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I got this car from my mom so I had the tires replaced then went to have the transmission flushed today. I didn't notice anything after getting the tires changed, but as I was leaving the mechanic where I had my transmission flushed, the Service Engine light was on and remained lit, so I went back to ask about it.

He checked the onboard computer, and it said something about a 505 error, Idling problems, and that my car didn't like something about the way it was idling. He turned off the Service light and said that if the light returned, I should go to the dealer to have it serviced. I did notice that as I slowed down to turn, the engine felt really weak and shook, and when I was at a red light, after a minute or so, the car started to vibrate and shake a little bit. Also, when I was parking, as I was reversing, the car could not move back without me having to give it more gas, when before today I could just let it move on its own.

What does this problem sound like? I didn't have these issues before today, before I had my car fixed up. Is there anything that I can do to try to solve any of these issues? I'm not really a car person, but I'd like to try a little myself first since having all four tires changed at once and having the transmission flushed put quite a sting in my pocketbook. Thanks.